The field of this invention relates in general to tubular joints, particularly subsea wellhead housings and wellhead connectors, and in particular to a seal assembly that provides sealing if the wellhead housing conical sealing surface becomes damaged.
A subsea well has a wellhead housing located at the subsea floor. The wellhead housing is a tubular member having a bore. A wellhead connector is lowered from a vessel at the surface over the wellhead housing to connect the subsea well to the surface. The wellhead connector has a connection for connecting to the exterior of the wellhead housing. Thus, a wellhead is one specific type of a tubular joint which is often used in the oilfield.
The wellhead housing has an upward-facing shoulder on its upper end that is engaged by a downward-facing shoulder on the lower end of the wellhead connector. The wellhead housing has a conical upward-facing shoulder at its upper end. The wellhead connector has a conical downward-facing shoulder. The wellhead connector also has a recess located radially inward from the downward-facing shoulder.
A metal seal locates between the wellhead connector and the wellhead housing. The metal seal has a conical upper surface that seals against the conical surface of the wellhead connector. The metal seal has a lower conical surface that seals against the conical surface of the wellhead housing. A rib extends radially outward from the two conical surfaces for location in the recess.
While the metal seal works well, if the conical surface of the wellhead housing becomes damaged, problems occur. The metal seal will not seal against the damaged lower surface. The wellhead housing is cemented in the ground and connected to casing and conductor pipe. It is not possible to pull the wellhead housing from the subsea floor for redressing the conical sealing surface.
A prior design for addressing this problem are illustrated in U.S. Pat. No. 5,103,915. In this design, the subsea wellhead housing has a secondary sealing surface machined below its conical primary sealing surface during manufacturing. The secondary sealing surface extends downward and is of a greater diameter than the bore. A conventional metal seal locates between the wellhead housing and the wellhead connector. The conventional seal seals against the primary sealing surface of the wellhead housing. The secondary sealing surface is not used so long as the wellhead housing primary sealing surface is in good condition. If the wellhead housing primary sealing surface becomes damaged, then a second seal ring is utilized in lieu of the first seal ring. The second seal ring has a support surface that leads to a secondary surface. The secondary surface is cylindrical and is sized to seal against the secondary surface in the wellhead housing. The support surface on the second seal ring is sized so that it will be spaced by a slight gap from the damaged primary sealing surface of the wellhead housing. This prior art device claims that a good seal between the wellhead housing and the wellhead connector can be maintained without need to redress the wellhead housing primary sealing surface. In another embodiment, the secondary seal surface is disclosed as being conical rather than cylindrical and at a lesser angle relative to vertical than the primary sealing surface. This configuration provides for a primary conical sealing surface at one angle, leading into a secondary conical sealing surface at another angle.
The different configurations of the design just described are illustrated in FIGS. 2 and 4 of U.S. Pat. No. 5,103,915. The main problem with this design is that the primary sealing surface, when it fails, is usually eroded due to the velocity effects of leaking fluid. These erosive effects attack not only the primary sealing surface but also the adjacent secondary sealing surface which, looking in the direction of the leaking fluid, presents itself first so that the erosive effects wind up damaging not only the primary but the secondary sealing surfaces in the wellhead. Thus, in effect, the design depicted in U.S. Pat. No. 5,103,915 is not serviceable, even with a replacement gasket, since the secondary surface has irregularities from the erosive effects and can no longer create a seal with the gasket against the connector. This phenomenon is illustrated in FIGS. 1-3 of the present application which depict a prior design akin to that shown in U.S. Pat. No. 5,103,915. Referring to FIG. 1 of this application, the wellhead 10 is shown having a single sealing surface 12, which is tapered. Gasket 14 has a matching taper 16 so that it can be squeezed against the sealing surface 12 by the connector 18. A clamp, generally referred to as 20 and which is of a known design, secures the wellhead 10 to the connector 18 and at the same time, forcing the connector 18 down against the gasket 14 to press the tapered surface 16 of the gasket 14 hard against the sealing surface 12 on the wellhead. In this design, the internal pressure in bore 22 can over time develop a leakpath which begins adjacent the lower end 24 of the gasket 14 in the transition area between bore 22 and tapered surface 16. As fluid under pressure begins to escape past the gasket 14, it begins to erode away part of the tapered sealing surface 12 and, in the configuration of FIG. 1, portions of the wall defining bore 22.
An alternative known prior art design is illustrated in U.S. Pat. No. 5,103,915 and shown in FIGS. 2 and 3 of this application. In FIG. 2, the original gasket 26 is shown with its tapered surface 28 firmly against the tapered sealing surface 30 on the wellhead 32. As before, the connector 34 is clamped by clamp 36 to hold tapered surface 28 against the sealing surface 30 of wellhead 32. Sealing surface 30 is set to be the primary sealing surface, while an adjacent surface 38, which can be cylindrical or tapered, extends immediately below the primary sealing surface 30. During normal operations with an effective seal being formed between surfaces 28 and 30, the gasket 26 is not in contact with the secondary sealing surface 38. The intention of this design is to make use of secondary sealing surface 38 should leakage occur past sealing surface 30. The problem occurs when erosion damage, 20 which is shown in FIG. 3, begins near the lower end 40 of the primary sealing surface 30. As indicated by the cross-hatched area 42 in FIG. 3, the erosive effects spread to a significant portion of the secondary sealing surface 38. Thus, when an oversized replacement gasket, which extends further downwardly with the intent of sealing against the secondary surface 38 is installed in the wellhead 32, the result is unsatisfactory as the hoped for sealing surface 38 has been damaged by the fluid velocity leaking past gasket 26 at surface 30. Thus, the problem with the design shown in FIGS. 2 and 3 of this application is that the secondary sealing surface 38 is configured so that it is in harm""s way when the erosive effects of a leak begin. It, therefore, is not available as a smooth surface necessary to get reliable sealing with a replacement gasket made to bridge the damaged primary sealing surface 30 and further designed to seal up against the secondary sealing surface 38 which, at this time, is not serviceable.
Accordingly, it is an object of the present invention to configure a tubular connection, one example of which could be a wellhead, internally, so that in the event leakage past a gasket occurs, the secondary sealing surface is available for use in a serviceable condition, thereby allowing the leak to be repaired, despite the damage to the primary sealing area. By virtue of the proper configuration between the secondary and primary sealing surfaces, the configuration of the present invention allows for reliable use of a secondary or backup sealing surface in conjunction with a backup or contingency gasket configured to reach the secondary sealing surface. The conforming shape of the contingency gasket to the wellhead configuration is also one of the novel inventions disclosed.
Other related wellhead designs of the prior art are disclosed in U.S. Pat. Nos. 5,687,794; 5,039,140; 4,709,933; 4,563,025; 4,474,381; 4,214,763; 3,749,426; 3,556,568; and 3,507,506.
Those skilled in the art will better appreciate the scope of the present invention from a review of the description of the preferred embodiment below.
A tubular connection, an example of which is a subsea wellhead having a primary and secondary seal areas allows the use of a backup or contingency gasket for engagement with the secondary seal area in the wellhead should a failure occur in the primary seal area. In the preferred embodiment, the primary and secondary seal areas are sufficiently separated such that the erosion damage which occurs from leakage with the original gasket adjacent the primary seal area, which can spread below the primary seal area, leaves the secondary seal area unaffected. A backup or contingency gasket can be inserted for sealable contact with the secondary sealing area for further well operations.